Goods that are packed for the exposure in frozen-food or refrigerated display counters are usually provided with labels, suitably in a completing packing step. In addition to the denomination, origin, and other identification of the goods that may be pre-printed on the label, in said completing packing step there is printed or typed a bar-code that contains information about the package in question and the goods according to standardized article coding systems. The bar-code is usually supplemented by a row of digits, which, in the case of the European Article Numbering EAN 13, contains 13 positions. Upon payment of the goods, the bar-code is read in an optical way by means of a scanner. If the scanner for any reason cannot interpret the barcode, the cashier has to enter the combination of digits of the EAN-code manually in his/her computerized cash register, which slows down the flow of goods through the cash-counter and constitutes a disturbing interruption on an occasion of payment. The optical quality of the bar-code, foremost in respect of the contrasting effect of the code bars with the base colour of the label, is therefore of great importance to a smooth cash-counter flow. The most commonly occurring labels in this connection are made with a thermal-printable layer, which contains a colouring sub-stance that generates a visible print upon supply of heat. The print is achieved by means of a thermal printer that may be located in a packing and labelling equipment in which the labels are applied to the packages for goods.
Thermal-printable materials have a broad application within different application areas, and need no detailed presentation, per se. However, it may be mentioned that thermal-printable materials generally comprise a substrate that is coated or prepared with a colouring substance, or a combination of substances, that at a certain elevated temperature reacts in order to appear in a colour that contrasts with the base colour of the label. Such colouring substances and combinations of substances that trigger colouring reactions by the influence of temperature are well known. Typically, they may consist of pH-sensitive mixtures containing, for instance, precursors of azo dyes, co-reacting binding substances, and substances, which initiate an alkaline reaction, or colourless/pale leuco dyes, and proton-releasing substances, i.e., acids, which together form a contrasting colour when exposed to the elevated temperature, usually 60° C. or higher. The colouring reaction in such compositions of substances, in response to supplied heat, may be a direct result of the inherent thermodynamic properties of the mixture, or be caused by the activation of a thermally unstable reactant, or by a release of a reactant from a protecting encapsulation in a fusible matrix initiated by the supply of heat. Within said technical field, a variety of colouring compositions of substances and colouring substances, acid reactants and binding-matrix substances are known. Without limiting the invention to anyone or any combination of said compositions of substances, it should be mentioned that the literature contains comprehensive lists of known components and compositions that may be included in thermal-printable materials suitable for the practice of the invention. Among said examples of literature, U.S. Pat. No. 4,370,370; U.S. Pat. No. 4,535,347; U.S. Pat. No. 4,591,887; U.S. Pat. No. 4,898,849; U.S. Pat. No. 5,017,821; U.S. Pat. No. 5,288,688; and U.S. Pat. No. 5,354,724 may be mentioned.
A label having a temperature-monitoring function may be made from a label of the type generally described above, which is arranged to generate a visual indication in case the goods, during the handling between the labelling and sale at any instant of time, or during a predetermined total time, has been exposed to a higher storage temperature than the highest allowable one prescribed. In this connection, by visual indication, an indication recordable by an optical scanner as well as an indication visible to the naked eye may be understood. Upon reading of a bar-code by an optical scanner, a verification of the condition of the goods is provided automatically. In this way, quality controls can be carried out simultaneously with the reading of a bar-code for different purposes during the handling of the goods from producer to consumer, an immediate affirmation being obtained that the goods in question up to the reading having been handled under the prescribed temperature conditions as long as the bar-code is readable. Conversely, if the bar-code is not readable, an affirmation is obtained that the goods at some point in the handling chain have been exposed to temperatures that may have degraded the quality thereof.